Servo control systems that maintain the position of a magnetic head relative to tracks in magnetic storage media are well known. European Patent Application EP 0 690 442 A2, published Jan. 3, 1995, entitled “Servo Control System”, Albrecht et al., describes a servo control system having a magnetic recording head for writing servo position code across the width of the magnetic recording tape and down the length of the tape. The head also has multiple gaps in the down-the-tape direction. The head can write a pattern along the length of the tape wherein the pattern contains multiple gaps spaced across the width of the tape. The advantage of such a recording head is the ability to write servo code along the entire length of the tape in discrete areas (bands) across the entire width of the tape in a single pass. This increases position accuracy of one pattern with respect to another. This also provides an economic advantage because writing each band separately would be significantly more expensive.
As shown in FIG. 1, a typical prior art head 100 is fabricated from two parallel pieces of ferrite 110 separated by a layer of glass 120. The top surface of the ferrite-glass-ferrite head 100 is curved, defining the contact surface of the tape. This curved surface is covered first with a thin magnetic seed layer 140 and then later with a layer 130 of magnetic material several microns thick. Gaps 150 defining the writing pattern extend through the magnetic layer 130 to the seed layer 140 over the central glass region. A wire winding 160 with one or more turns passes through a groove 170 in the ferrite extending across the long axis of the head 100 next to the glass. Current passing through the wire 160 energizes magnetic fields in the gaps 150. The magnetic field writes the gap pattern on the passing tape (not shown). Contact between the passing tape and the head causes wear to the head thereby decreasing its life.
Using the prior art approach, each head 100 must be fabricated individually (i.e., discretely). The small size and shape of the discrete heads makes it difficult to apply a photoresist layer that has a uniform thickness. Forming a uniform photoresist layer is an especially important consideration because the photoresist is used to pattern the recording gaps. Photoresist thickness non-uniformity directly and adversely affects the quality of the patterns, especially for narrow gap dimensions. Such patterns require the formation of narrow lines (on the order of 1.5 μm). Also, because the surface is curved, one must compensate for the curvature of the head in the photolithographic process. It is particularly difficult to apply an even layer of photoresist to pattern the recording gaps on this curved surface. Applying resist by spinning is difficult in the case where the length-to-width ratio of a part is far greater than unity. Although resist can be dipped or sprayed onto the part, these methods are cumbersome and have not been refined for submicron geometric tolerances. Nor have they been efficiently adapted to fabrication of an array of heads.
To produce this rounded shape, the heads must be individually machined although they could be produced in bar form to reduce fabrication costs. FIG. 2 illustrates a bar 200 containing several heads 100. Lines 210 perpendicular to the long axis indicate where the bars could be sawed apart to form individual heads 100. Although the bars 200 can be cut up into several heads 100, the procedure is still not very efficient and provides identical or even more difficult challenges to achieving photoresist uniformity. Thus, it can be seen that multiple gap servo write heads are expensive to manufacture and have limited servo pattern definition.
Another prior art approach attempts to solve the problem of producing the servowriter heads individually. This approach is described in U.S. Pat. No. 6,018,444. According to the process, existing wafer processing techniques are used for batch operation of head production. FIG. 3 is a drawing illustrating a cross section of a servo write head 300 with recording tape 310 passing over the head 300 according to the prior art. The tape 310 first contacts the head 300 on the curved surface 320 located at the leading edge of the head 300. Tape 310 continues over the flat surface 330 supported by an air bearing which has formed between the tape 310 and the head surface 330. The tape 310 continues to the opposite edge of the head, exiting the head over a trailing curved edge 350 similar to the entrance. The tape passes over the rounded edges and proceeds over the flat portion 330 of the head 300 separated from the surface by a thin air bearing. A spacer 340 region has a non-magnetic material (such as glass) that separates two pieces of ferrite 360. External inductive coil windings 160 are wrapped around either side of bottom portion 370 of head 300 to create the magnetic flux.
One drawback to this approach is that it relies on an air bearing surface for wear resistance. Further, the prior art air bearing must be tailored to the tape velocity. Small head-tape spacing is preferred for good pattern transfer. Because the presence of an air bearing surface can cause write errors, the air bearing for the head 300 in FIG. 3 must be tailored to the tape velocity. Another disadvantage of the prior art process is the difficulty in manufacture and assembly due to its multi-component design, curved shape, and wire winding.
Therefore, an unresolved need exists for a batch fabrication technique that will increase the manufacturing efficiency of servo write heads and also improve servo pattern definition for fine features while reducing head wear.
A further need exists for a one-piece construction not using an external winding, thereby simplifying the assembly process. What is also needed is a head that does not rely on an air bearing for wear resistance, but rather has a flat contour and a sharp air-skiving edge that result in contact recording which is insensitive to tape velocity. Yet another need exists for a batch fabrication technique that allows manufacture of an array of heads.